21 - THE COVALENT BOND REEXAMINED

Abstract

This chapter discusses two theories of covalent, namely, valence bond theory and molecular orbital theory. Both the theories arise from quantum mechanics and deal with the discrete electron energy levels and orbitals. The chapter describes how each theory explains single and multiple covalent bonds. It further describes the concepts of hybridization of atomic orbitals and delocalized electrons. The chapter presents molecular orbital energy diagrams for some simple diatomic molecules and for metals. It discusses how some of the properties of those species are accounted for by the molecular orbital theory and explains the concept of hybridization. Valence bond theory explains bond formation as the overlap of atomic orbitals. The greater the amount of overlap, the stronger is the bond. The possibility for bond formation exists when two atomic orbitals of similar energy level can overlap. Those orbitals must be occupied by the two electrons needed for the covalent bond. Molecular orbital theory explains bond formation as the occupation by electrons of orbitals characteristic of the whole molecule. The chapter illustrates the bond formation by atomic orbital overlap by the H 2 molecule. Molecular orbital theory provides a way of calculating bond order. The size of the forbidden energy gap between the valence and conduction band plays an important role in the differences among conductors, semi­conductors, and insulators.